Aerobic activity parameters such as the rate of oxygen consumption (VO2), maximal oxygen consumption (VO2 max), and energy expenditure (EE) reflect the extent of cardiorespiratory fitness of an individual. Measurements of these parameters is typically done in controlled laboratory settings, in which the composition of the exhaled breathes can be analyzed. Estimations of aerobic activity parameters may be done using other physiological signals such as the heart rate, along with additional data providing indications of factors such as the exercise being performed and physiological properties of the individual (e.g., age, weight, body composition, etc.)
While it is known that measurements of the respiration rate can enhance the accuracy of estimation of aerobic activity parameters, monitoring a person's respiratory parameters while exercising in day-to-day settings can be difficult due to the person's movements and activities. Additionally, when used for long periods (e.g., several hours each day), some current approaches for measuring some respiratory parameters may be impractical (e.g., a spirometer) and/or uncomfortable (e.g., systems involving chest straps). Thus, there is a need to be able to monitor a user's respiration in a comfortable way in ordinary day-to-day settings, and to use such measurements in order to calculate aerobic activity parameters.